This invention relates in general to apparatus for working sheet material and deals more particularly with improvements in apparatus of the type which includes a sheet material supporting surface and a tool carriage assembly for moving a tool in working relation to sheet material spread on the supporting surface in response to command signals received from a programmable computer.
In a machine of the aforedescribed general type, the programmed movements of the tool are relative to the supporting surface. Accordingly, in order to achieve accuracy in the finished work product it is essential that the sheet material be held in firmly fixed position relative to supporting surface. In a machine of the type wherein the tool moves in direct working engagement with the sheet material, as, for example, in a machine where the tool is a cutting tool or a marking tool, it is highly desirable that the material be held firmly against the supporting surface in the region of the tool so that working pressure exerted upon the material by the tool does not cause the material to shift relative to the supporting surface.
Heretofore various apparatus for working on sheet material has been provided which includes means for holding sheet material in engagement with a supporting surface in proximity to a working region of a tool. Vacuum holddown tables have proven particularly effective for this purpose, however, such tables are relatively expensive to produce and require vacuum producing equipment, which adds materially to the construction and operational costs of the equipment.
Accordingly, it is the general aim of the present invention to provide improved programmable apparatus for working on sheet material and which includes relatively simple magnetic or electromagnetic means for firmly holding sheet material in fixed position relative to a supporting surface and in close proximity to the operating region of a tool which works upon the sheet material.